(i) Field to which the Invention Relates
The invention is with respect to a process for making a thermal insulating body from highly dispersed insulating material, opacifier, reinforcing fiber mix and, if desired a binder, these materials being mixed together and compacted.
(ii) The Prior Art
In an earlier suggestion (see German Auslegeschrift specification No. 1,671,186) for producing thermal insulating material, a mix, based on aerogel particles and inorganic fibers, is formed into the desired structures. This mix is made up of silicon dioxide or diatomaceous earth, aerogel particles and ceramic aluminum silicate or carbon fibers or mixes thereof. For the mixing operation, these materials are placed in a vessel having a turning paddle near its lower wall. After shutting the vessel, the material is swirled round by the blades of the paddle and so completely mixed. In a preferred form of the invention, an opacifier undergoes addition to the aerogel and fiber material before mixing. Nextly, all the materials undergo mixing in a vessel with a turning paddle.
As a rule such silica-aerogel particles have a diameter under 1 micron and because of their high-bulk structure have a low density. Because such a thermal insulating material is made up for at least 50% of such an aerogel, nearly the full space taken up by the body produced of such thermal insulating material is filled with aerogel particles and the air round them, the fiber materials and the opacifier hardly taking up any of the volume.
In mixing tests, it has turned out that it is very hard to make up a mix containing 50% of aerogel particles and containing, furthermore, reinforcing fiber material and/or opacifier. In fact, if the addition is made of relatively small amounts of opacifier and/or fiber material to a great amount of aerogel material, no evenly mixed product will be produced even after stirring or agitating for a number of hours. In fact, complete mixing is only possible by the turning paddle in a limited part of the vessel while the material over the paddle is only moved very slowly downwards to undergo complete mixing. It may frequently be the case that the paddle, turning at a high speed, will be heated by the mixing operation while, on the other hand, not being able to undertake the mixing operation with the desired outcome. Such an unhomogeneous mixing operation naturally makes for a lower quality of the end product, its coefficient of thermal conduction and heat reflexion properties being decreased.